The present invention relates generally to the field of data storage systems, and more particularly to detecting damage to tunneling magnetoresistance (TMR) sensors.
In magnetic data storage systems, data is written to and read from magnetic recording media using magnetic transducers. TMR sensors are one type of magnetic transducer that are used in magnetic data storage systems (e.g., hard disk drives). TMR sensors comprise two ferromagnetic layers separated by a thin oxide tunnel barrier (e.g., MgO or AlO). The polarization direction of one of the ferromagnetic layers is “pinned” to a particular direction, while the polarization direction of the other ferromagnetic layer (i.e., the free layer) is free to align with the polarization direction of magnetic storage media over which the TMR sensor is passed. When the polarization directions of the free and pinned layers which straddle the tunnel barrier are parallel, the electrical resistance across the TMR sensor is low because it is more likely that electrons will tunnel through the tunnel barrier; when the polarization directions of the two ferromagnetic layers are antiparallel, the electrical resistance across the TMR sensor is high because it is less likely that electrons will tunnel through the tunnel barrier. Accordingly, TMR sensors can read bits stored on magnetic storage media by measuring changes in resistance across the TMR sensor due to the magnetic fields generated by the bit transitions as the sensor passes over those bits.
Damage to TMR sensors can occur in numerous ways and is often difficult to detect. For example, damage due to dielectric breakdown across the tunnel barrier, pin-holes, and lapping smears can all result in degraded and/or abnormal amplitude response of a TMR sensor. While magnetic tests can often detect the damage by degraded or abnormal magnetoresistance (MR) response from a magnetic field, it can be difficult or impossible to subject the TMR readers to magnetic fields in the manufacturing line.